As characteristics of a substance being measured, for example, an electric conductivity is used as, in particular, a scale for measuring a concentration of an ion capable of moving in an aqueous solution, and an electric conductivity measuring instrument is used for measurement of ion concentration of many aqueous solutions. Generally, the electric conductivity measuring instrument determines increase/decrease of ion concentration of an aqueous solution being measured by measuring a resistance between a detection electrode and an electrode for supplying a current from a power source.
In a case where a variation of electric conductivity or a difference in electric conductivity between a plurality of positions being measured is determined by using a conventional electric conductivity measuring instrument, when the variation or the difference is small as compared with an absolute value of the electric conductivity being measured, because the measurement range of the instrument is being adjusted for a relatively large absolute value of electric conductivity, the measurement of a fine variation or difference is very difficult, or the measurement data are very poor in reliability. In practice, however, there are much demands for determining such a fine difference or variation between two or more measurement points different from each other in position or in time, and if such a fine difference or variation can be measured with a high reliability and with high accuracy and sensitivity, such measurement would find wide application.
Accordingly, in order to satisfy the above-described demands, the applicant of the present invention previously proposed a multi-dimension electric conductivity measuring instrument in JP-A-2001-311710 as an instrument capable of abstracting and determining a variation of characteristics of a substance being measured such as an aqueous solution with a high accuracy. This multi-dimension electric conductivity measuring instrument comprises at least two electric conductivity measuring cells each having at least two electrodes coming into contact with a substance being measured, and the electric conductivity measuring cells are electrically connected to each other so that the detection signals themselves from the respective electric conductivity measuring cells are processed by at least addition or subtraction.
In this instrument, to the detection signals themselves from the respective electric conductivity measuring cells, namely, to the detection signals themselves abstracted simultaneously, the electrical processing such as addition or subtraction is carried out, and the signals after the processing, being amplified as needed, are outputted as a difference or a variation between measured electric conductivities of the respective electric conductivity measuring cells. Because a difference between detection signals abstracted simultaneously is outputted, it becomes possible to detect a variation with a high S/N ratio by erasing a noise commonly generated in the respective electric conductivity measuring cells, and it becomes possible to output only the difference or the variation at a high accuracy, by amplification, etc. Therefore, in this instrument, unlike an arrangement wherein a plurality of conventional electric conductivity measuring instruments are disposed and a difference or variation between the data measured therefrom is obtained, a fine difference or variation between electric conductivities of a plurality of measurement points different in position or in time from each other can be determined with a high reliability and with high accuracy and sensitivity.
Although the invention disclosed in the above-described JP-A-2001-311710 was proposed for measurement of electric conductivity, the technology by which a difference or variation in characteristics to be measured is determined with a high reliability and with high accuracy and sensitivity by outputting a difference between detection signals simultaneously taken out from at least two sensors, can be applied basically to measurement of any characteristic.
However, the indicated value in the multi-dimension electric conductivity measuring instrument proposed by the above-described JP-A-2001-311710 is a fine difference or variation between electric conductivities in a plurality of measurement points different in position or in time from each other, and such an indicated value is not an absolute value of electric conductivity. In practice, however, a variation in absolute value of electric conductivity is frequently required for measurement of variation in concentration of impurities in an aqueous solution, etc. Even when a characteristic other than the electric conductivity is determined, measurement of absolute value of the characteristic is frequently required.